1. Field of the Invention
The present invention relates to an image processing apparatus, and more particularly to optimum processing of a color image.
2. Related Background Art
Full-color images have been recently used extensively not only in printing and design offices but also in general business offices. In order to read and output such color images with high fidelity, there have been developed various types of color copying apparatus. Color copying apparatus are required to output a full-color image at high gradation, and also to output a color character document clearly and at high resolution. As a method of outputting a full-color image at a high gradation, there are known a dither method, a screen dot modulation method and the like. If these methods are used for character images or line images, the resolution is considerably degraded and the quality of character is worsened. A binarization process is suitable for fairly reproducing character images and line images. It is well known, however, that if the binarization process is used for screen dot images or photographic images, the gradation and image quality are considerably degraded. Various methods have been proposed to satisfy both character quality and halftone quality. For example, (1) according to Japanese Patent Laid-Open Publication No. 61-11719, character quality is improved by making a black (BK) signal large and making yellow (Y), magenta (M) and cyan (C) signals small, based on the BK signal amplitude calculated from color-separation original Y, M and C signals, so that at the edge portion of a black character, the respective color components are replaced with a black component as much as possible; (2) there is also an attempt to improve character quality by emphasizing all edges or contours of an image without degrading halftone; and (3) according to another method, high resolution image processing and high gradation image processing are selectively used by designating a character area, a screen dot area and a photographic area through manual input by an operator.
With the first method, the contour of a black character can be represented by black color only. However, hairs or eyelashes are sometimes erroneously judged as a black character, or a superposition of yellow, magenta and cyan screen dots is erroneously judged as a black character resulting in formation of an unnecessary black dot. Therefore, the first method is not satisfactory from the image quality viewpoint. With the second method, the sharpness of image is improved. However, since a black character portion is obtained by four-color superposition, color aberration occurs and the character quality is not satisfactory. With the third method, each image area can be processed independently. For example, the image area designated as a black character area is processed with high resolution using only black color, and the color halftone area is processed with excellent halftone. However, an operator is required to precisely designate each image area position, necessitating very complicated operator work.